Postgres data changes
visibility rule: during a query execution, data changes made by the
query itself (via SQL-function, SPI-function, triggers) are
invisible to the query scan. For example, in query

INSERT INTO a SELECT * FROM a;

tuples inserted are invisible for SELECT scan. In effect,
this duplicates the database table within itself (subject to unique
index rules, of course) without recursing.

But keep in mind this notice about visibility in the SPI
documentation:

Changes made by query Q are visible by queries which are
started after query Q, no matter whether they are started inside
Q (during the execution of Q) or after Q is done.

This is true for triggers as well so, though a tuple being
inserted (tg_trigtuple) is not visible to queries in a BEFORE
trigger, this tuple (just inserted) is visible to queries in an
AFTER trigger, and to queries in BEFORE/AFTER triggers fired after
this!